There are a variety of factors that contribute to the human body's weakness at high altitudes. Our bodies were not created to handle the incredibly frigid temperatures, the high winds, or the lack of oxygen that are common in places such as these. Air temperature decreases with height, meaning that you will be much colder on the summit of an 8000-meter peak than you would when sitting on the beach in Miami.
One common myth is that it is a decrease in the amount of oxygen that acts as the driver for all types of altitude sickness. The true cause, however, is the lack of atmospheric pressure. The air molecules are much more dispersed at higher altitudes, so there is actually much less atmosphere up there (contributing to the significant risk of sunburn), not just less oxygen! There is actually the same concentration of oxygen in the atmosphere regardless of elevation. The reduced pressure does not allow as much air into your lungs, which can therefore not absorb as much oxygen as your body needs.
Atmospheric pressure determines how much air is "pushed" into the lungs with each breath.
The atmospheric pressure at the summit of Mount Everest is about a third that at sea level.
Heart beat rate, breathing rate, and the production of red blood cells, which carry oxygen throughout the body, increases as the body acclimates to high altitude allowing more oxygen to be "grabbed" from every breath.